A Scanning Transmission Electron Microscope (STEM) supported by the NIH Biotechnology Resources Program is available for use by qualified biomedical scientists. Projects include: mass measurement of single molecules, development of single and multiple heavy atom stains, imaging of frozen specimens, imaging and motion of single heavy atoms as a function of temperature, digital image analysis, and instrumentation. Biological structure studies in progress include: filamentous phage fd, purple membrane, adenovirus DNA-protein complexes, muscle proteins, SV40 replication, repair of UV damaged DNA, cyanobacterial phycobilisomes, chromosome fibers, inverted repeats in mouse DNA, muscle regulatory proteins, fibrinogen, xanthan polymers, collagen, mammalian ribosomes, lac repressor, anti-tumor platinum drugs, menigopolysaccharides, nucleosome particles, and SV40 mini chromosome. The Brookhaven STEM is a state-of-the-art microscope offering 2.5A resolution at 50 KV on specimens observed at temperatures between 130 degrees below centigrade and 100 degrees centigrade. Facilities are provided for freeze drying with vacuum transfer to the STEM, and for transfer and observation of frozen hydrated specimens. Use of the cold stage significantly reduces atom motion and mass loss and permits observation of frozen hydrated specimens. Low dose, quantitative STEM imaging of unstained or selectively labeled objects has been shown to give the mass profile of the object.